View source for Running a HP-DFT calculation with CP2K

This tutorial shows how to set up and run a Hairy Probes (HP) <ref> M. G. Zauchner, A. P. Horsfield, and T. N. Todorov, Efficient electron open boundaries for simulating electrochemical cells, Physical Review B 97, 0.1103/PhysRevB.97.045116 (2018) </ref> DFT calculation using the [https://www.cp2k.org/  CP2K2022.1] computational package. Our implementation of Hairy Probes within the CP2K code is presented in ref. <ref> M. Buraschi, A. P. Horsfield and C.S. Cucinotta, Revealing Interface Polarization Effects on the Electrical Double Layer with Efficient Open Boundary Simulations under Potential Control, J. Phys. Chem. Lett. 2024, 15, 4872−4879</ref>. The module required to run a HP-DFT calculation is still not part of the official release of CP2K; this tutorial shows how to download and compile CP2K2022.1 with the necessary HP-DFT module.

= Compiling CP2K2022.1 with Hairy Probes =
1. Download CP2K2022.1 and set up the required libraries: 
 
   git clone -b support/v2022.1 https://github.com/cp2k/cp2k.git ~/${INSTALL_DIR}/cp2k


2. Download the Hairy Probes modules:

   git clone https://gitlab.doc.ic.ac.uk/rgc/hairy_probes_dft_cp2k.git ~/${INSTALL_DIR}/Hairy_probes_modules

3. Copy the Hairy Probes modules (all the modules contained in the <code>Hairy_probes_modules</code> repository) in the source code folder of CP2K:

   cp ~/${INSTALL_DIR}/Hairy_probes_modules/* ~/${INSTALL_DIR}/cp2k/src

4. Continue compiling CP2K2022.1 per usual. For example:

   cd ~/${INSTALL_DIR}/cp2k/tools/toolchain/
   ./install_cp2k_toolchain.sh --no-check-certificate --with-elpa=no 
   source ~/${INSTALL_DIR}/cp2k/tools/toolchain/install/setup
   cp ~/${INSTALL_DIR}/cp2k/tools/toolchain/install/arch/* ~/cp2k/arch/
   cd ~/${INSTALL_DIR}/cp2k 
   make -j 24 ARCH=local VERSION="ssmp sdbg psmp pdbg" &> make.log

(This example shows how to compile CP2K2022.1 using the provided <code>install_cp2k_toolchain.sh</code> script, which builds the necessary libraries and architecture files. Any other strategy to compile the code, e.g. easybuild, is equally valid. The important point is to add the necessary modules to the source code before compiling). 

= Run an HP-DFT calculation =

To run an HP-DFT calculation the following section needs to be added under the <code>DFT</code> section of the input:

     &HAIR_PROBES 
       T 300
       MU 0.500000
       ATOM_IDS 1..54 
       EPS_HP 1.0E-5
     &END HAIR_PROBES
     &HAIR_PROBES 
       T 300
       MU -0.500000     
       ATOM_IDS 347..1045
       EPS_HP 1.0E-5
     &END HAIR_PROBES
     &HAIR_PROBES 
       ALPHA 0.01
       T 300
       MU 0.000000     
       ATOM_IDS 193..543
       EPS_HP 1.0E-5
     &END HAIR_PROBES

The '''keywords''' are defined as follows:
*<code>T</code>: electronic temperature of the electrons in the reservoirs;
*<code>MU</code>: electrochemical potential of the electrons in the reservoirs;
*<code>ATOM_IDS</code>: indices of the atoms in the system coupled with hairy probes;
*<code>ALPHA</code>: parameter. A value of <code>ALPHA</code> lower than 1.00 defines a set of probes as 'solution probes';
*<code>EPS_HP</code>: defines the accuracy of the HP calculation.   

You can repeat the section <code>HAIR_PROBES</code> for as many sets of probes as you want in your calculation. Generally, you want to set <code>ALPHA</code> to be very low for atoms in solution. You can use <code>HAIR_PROBES</code> both for <code>GEO_OPT</code> (geometry optimization) and <code>MD</code> (molecular dynamics) type calculations (as long as the force evaluation (<code>FORCE_EVAL</code>) is done using DFT, of course). 

Other '''important''' things to know:
*To run HP-DFT you need to remove the <code>SMEARING</code> section;
*As of now, the HP-DFT implementation only works with <code>STANDARD DIAGONALIZATION</code> (it does not work with <code>OT</code>, for example).


= Example Calculation: Parallel-Plate Capacitor =
[[File:System and probes.png|thumb|400px|Representation of the parallel-plate capacitor model for a HP-DFT calculation]]
This example shows an HP-DFT calculation performed for a straightforward parallel-plate capacitor model. The model consists of two 3-layer Pt(111)(6x6) slabs separated by 10Å of vacuum. The slabs were built using a conventional unit cell with a lattice parameter of 3.97 Å. The supercell is periodic in x and y (directions parallel to the slabs' surfaces) but not in z (directions perpendicular to the slabs' surfaces), where an implicit Poisson solver was used <ref> M. H. Bani-Hashemian, S. Bruck, M. Luisier, and J. VandeVondele, A generalized poisson solver for firstprinciples device simulations, J Chem Phys 144, 044113 (2016). </ref> . The calculation was performed at the Γ-point. The PBE functional <ref> J. P. Perdew, K. Burke, and M. Ernzerhof, Generalized gradient approximation made simple, Phys Rev Lett 77, 3865 (1996). </ref> in combination with Grimme's D3 correction <ref>S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, A consistent and accurate ab initio parametrization of density functional dispersion correction (dft-d) for the 94 elements h-pu, J Chem Phys 132, 154104 (2010). </ref> for dispersion interactions were employed. The valence electrons of the Pt atoms were described using the TZVP-MOLOPT basis set <ref>J. VandeVondele and J. Hutter, Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases, J Chem Phys 127, 114105 (2007). </ref>, while the core electrons were described with a GTH pseudopotential <ref>S. Goedecker, M. Teter, and J. Hutter, Separable dualspace gaussian pseudopotentials, Phys Rev B Condens Matter 54, 1703 (1996). </ref>.  
== HP-DFT calculation setup ==
For this calculation, two sets of main probes were coupled with the outermost layer of the capacitor model ('''atoms 1 to 36''' in the left slab and '''atoms 181 to 216''' in the right slab). Electrochemical potentials '''Δμ<sub>L</sub> = 0.5 eV''' and '''Δμ<sub>R</sub> = -0.5 eV''' were imposed on the left slab and the right slab respectively. This gave an overall electrochemical potential difference Δμ = |Δμ<sub>L</sub> - Δμ<sub>R</sub>| = 1 eV across the two plates. The '''electronic temperature''' was set at '''300K'''. Since there is no electrolyte in this system, solution probes were not required. The section controlling the HP calculation in the CP2K input therefore is: 

     &HAIR_PROBES 
       T 300
       MU 0.500000
       ATOM_IDS 1..36
       EPS_HP 1.0E-5
     &END HAIR_PROBES
     &HAIR_PROBES 
       T 300
       MU -0.500000     
       ATOM_IDS 181..216
       EPS_HP 1.0E-5
     &END HAIR_PROBES

The same calculation was repeated using Δμ<sub>L</sub> = 2 eV and Δμ<sub>R</sub> = -2 eV to give Δμ = |Δμ<sub>L</sub> - Δμ<sub>R</sub>| = 4 eV

== Results ==
In this section, a summary of the relevant results is reported. For more details check the full publication 

=References=
<References>